How can I add two textures?
Instead of using the Math node for adding, you should use the Math node with Multiply. So for example then the areas where your Musgrave texture is 0 will remove the influence of the Noise texture there.

If you use colorful noise you could have a color mix node with that noise and white (or whatever color you want when the musgrave is 0) rbg color as inputs, and the mix factor would be controlled by the musgrave texture value